I check caniuse often, and I find that most of the mainstream APIs I want to use Safari supports.

Is there some specific glaring hold back that I'm missing?

Most of the new daily use APIs seem to be well supported, not to mention iOS Users tend to be extremely well update to date (at least in my experience, and it seems to be the case as they report often in their developer conferences). I tend to note that Mac users aren't usually 1 or 2 releases behind, either (and they have been getting better at back porting Safari versions)

Here's what I see:

https://caniuse.com/#compare=safari+10.1,safari+11,safari+11...

and I don't see any glaring omissions of day to day APIs that I think most developers use most often, since not everyone is even on the latest versions of Chrome, per se, or the need for enterprises to mandate support for IE11.

I can see:

Lazy Loading of images and iframes via an attribute

requestIdleCallback API

The `:has` and `:focus-visible` pseudo selectors
`text-justify` is a weird omission, I will say that
(though its roadmapped to arrive)

They have ResizeObserver in 13 and the Tech preview, and
likely this will land with iOS 13 at some point. Desktop
Safari and iOS Safari don't tend to be too far behind
each other.

CSS Paint API (Then again, only Chromium based browsers
have this right now anyway, this is essentially houdini.
Also worth noting it can be enabled behind a flag in
developer versions of Safari right now, so I wholly
expect this to be implemented in the future. FireFox
isn't even that far along yet)

I'm sure I could find others, but how many day to day APIs are actually missing? All the big ones (CSS-Grid, Web components, CSS and SVG Animations etc) are alive and well, and the Safari team has most of the missing ones in their pipeline.

I am open to discussing this further, of course! I want to know if I'm missing something or unique, and I realize not every case is the same.

I also acknowledge that No, its not as good as Chrome, and some of these features have to be polyfilled (which is nice that it can be, but not ideal), and yes, I wish they were faster (much faster) at updating webkit and safari across the board. I'm not here to sing the praises of their release schedule and feature updates by any means.

However IE never had a real history of implementing things that most developers even wanted to use most of the time. It was truly that bad. With the exception of Flex Box and Ajax, I can't think of anything that came out of Internet Explorer that became a standard or inspired a standard. Microsoft at many points was actively hostile in participating with much of the WHATWG/W3C process. IE11 even didn't support the ES6 standards nearly at all (They got Map, but not all of Map, but didn't implement Promises etc). That was just plain horrible in my opinion.

I think Safari is falling behind in some aspects, but in terms of covering most of the popular, meaningful APIs? they seem to keep up OK, much better than IE or even Edge ever did.

Its also important to acknowledge that Apple is a devices and software integration company, not a Web Application Company, so Safari follows a product schedule, not a rolling release one, though they have made a lot of motions to decouple the Safari update and release process from this product cycle its been on (slow, for sure)

I think they don't deserve this moniker, because its completely distracting from real criticisms that can be leveraged into action. By declaring it the new IE you're taking power away from better, more substantial, valid criticisms and concerns that I think the webkit/safari team would be more willing to engage, honestly.