For the lazy:

NEW FEATURES

extSoftVersion() now reports the version (if any) of the readline library in use.
The version of LAPACK included in the sources has been updated to 3.6.1, a bug-fix release including a speedup for the non-symmetric case of eigen().
Use options(deparse.max.lines=) to limit the number of lines recorded in .Traceback and other deparsing activities.
format(<AsIs>) looks more regular, also for non-character atomic matrices.
abbreviate() gains an option named = TRUE.
The online documentation for package methods is extensively rewritten. The goals are to simplify documentation for basic use, to note old features not recommended and to correct out-of-date information.
Calls to setMethod() no longer print a message when creating a generic function in those cases where that is natural: S3 generics and primitives.

INSTALLATION and INCLUDED SOFTWARE

Versions of the readline library >= 6.3 had been changed so that terminal window resizes were not signalled to readline: code has been added using a explicit signal handler to work around that (when R is compiled against readline >= 6.3). (PR#16604)
configure works better with Oracle Developer Studio 12.5.

UTILITIES

R CMD check reports more dubious flags in files ‘src/Makevars[.in]’, including -w and -g.
R CMD check has been set up to filter important warnings from recent versions of gfortran with -Wall -pedantic: this now reports non-portable GNU extensions such as out-of-order declarations.
R CMD config works better with paths containing spaces, even those of home directories (as reported by Ken Beath).

DEPRECATED AND DEFUNCT

Use of the C/C++ macro NO_C_HEADERS is deprecated (no C headers are included by R headers from C++ as from R 3.3.0, so it should no longer be needed).

BUG FIXES

The check for non-portable flags in R CMD check could be stymied by ‘src/Makevars’ files which contained targets.
(Windows only) When using certain desktop themes in Windows 7 or higher, Alt-Tab could cause Rterm to stop accepting input. (PR#14406; patch submitted by Jan Gleixner.)
pretty(d, ..) behaves better for date-time d (PR#16923).
When an S4 class name matches multiple classes in the S4 cache, perform a dynamic search in order to obey namespace imports. This should eliminate annoying messages about multiple hits in the class cache. Also, pass along the package from the ClassExtends object when looking up superclasses in the cache.
sample(NA_real_) now works.
Packages using non-ASCII encodings in their code did not install data properly on systems using different encodings.
merge(df1, df2) now also works for data frames with column names "na.last", "decreasing", or "method". (PR#17119)
contour() caused a segfault if the labels argument had length zero. (Reported by Bill Dunlap.)
unique(warnings()) works more correctly, thanks to a new duplicated.warnings() method.
findInterval(x, vec = numeric(), all.inside = TRUE) now returns 0s as documented. (Reported by Bill Dunlap.)
(Windows only) R CMD SHLIB failed when a symbol in the resulting library had the same name as a keyword in the ‘.def’ file. (PR#17130)
pmax() and pmin() now work with (more ?) classed objects, such as "Matrix" from the Matrix package, as documented for a long time.
axis(side, x = D) and hence Axis() and plot() now work correctly for "Date" and time objects D, even when “time goes backward”, e.g., with decreasing xlim. (Reported by William May.)
str(I(matrix(..))) now looks as always intended.
plot.ts(), the plot() method for time series, now respects cex, lwd and lty. (Reported by Greg Werbin.)
parallel::mccollect() now returns a named list (as documented) when called with wait = FALSE. (Reported by Michel Lang.)
If a package added a class to a class union in another package, loading the first package gave erroneous warnings about “undefined subclass”.
c()‘s argument use.names is documented now, as belonging to the (C internal) default method. In “parallel”, argument recursive is also moved from the generic to the default method, such that the formal argument list of base generic c() is just (...).
rbeta(4, NA) and similarly rgamma() and rnbinom() now return NaN‘s with a warning, as other r<dist>(), and as documented. (PR#17155)
Using options(checkPackageLicense = TRUE) no longer requires acceptance of the licence for non-default standard packages such as compiler. (Reported by Mikko Korpela.)
split(<very_long>, *) now works even when the split off parts are long. (PR#17139)
min() and max() now also work correctly when the argument list starts with character(0). (PR#17160)
Subsetting very large matrices (prod(dim(.)) >= 2^31) now works thanks to Michael Schubmehl’s PR#17158.
bartlett.test() used residual sums of squares instead of variances, when the argument was a list of lm objects. (Reported by Jens Ledet Jensen).
plot(<lm>, which = *) now correctly labels the contour lines for the standardized residuals for which = 6. It also takes the correct p in case of singularities (also for which = 5). (PR#17161)
xtabs(~ exclude) no longer fails from wrong scope, thanks to Suharto Anggono’s PR#17147.
Reference class calls to methods() did not re-analyse previously defined methods, meaning that calls to methods defined later would fail. (Reported by Charles Tilford).
findInterval(x, vec, left.open = TRUE) misbehaved in some cases. (Reported by Dmitriy Chernykh.)