A sibling comment mentions the reload4j project, so clearly someone thought that 1.x wasn't adequate to a degree of creating a new project around maintaining a fork. Can't speak of the project itself, merely the fact that its existence supports the idea that EOL software is something that people would prefer to avoid, even if they decide to maintain a backwards compatible fork themselves, which is great to see.

Here's a bit more information about some of the vulnerabilities in 1.x, someone did a nice writeup about it: https://www.petefreitag.com/item/926.cfm

I've also dealt with 1.x having some issues with data loss, for example, https://logging.apache.org/log4j/1.2/apidocs/org/apache/log4... which is unlikely to get fixed:

DailyRollingFileAppender has been observed to exhibit synchronization issues and data loss.

(though at least in regards to that problem, there are alternatives; though for the most part EOL software implies that no further fixes will be available)

But at the end of the day none of it really matters: those who don't want to upgrade won't do so, potential issues down the road (or even current ones that they're not aware of) be damned. Similarly, others might have unpatched versions of 2.x running somewhere which somehow haven't been targeted by automated attacks (yet) and might continue to do so while there isn't proper motivation to upgrade, or won't do so until it will be too late.

Personally, i dislike the idea of using abandoned software for the most part, when i just want to get things done - i don't have the time to dance around old documentation, dead links, having to figure out workarounds for CVEs versus just using the latest (stable) versions and letting someone else worry about it all down the road. Why take on an additional liability, when most modern tooling and framework integrations (e.g. Spring Boot) will be built around the new stuff anyways? Though thankfully in regards to this particular case slf4j gives you more flexibility, but in general i'd prefer to use supported versions of software.

I say that as someone who actually migrated a bunch of old monolithic Spring (not Boot) apps to something more modern when the versions had been EOL for a few years and there were over a hundred CVEs as indicated by automated dependency/package scanning. It took months to do, because previously nobody actually cared to constantly follow the new releases and thus it was more akin to a rewrite rather than an update - absolute pain, especially that JDK 8 to 11 migration was also tacked on, as was containerizing the app due to environmental inconsistencies growing throughout the years to the point where the app would roll over and die and nobody had any idea why (ahh, the joys of working with monoliths, where even logs, JMX and heap dumps don't help you).

Of course, after untangling that mess, i'd like to suggest that you should not only constantly update packages (think every week, alongside releases; you should also release often) but also keep the surface area of any individual service small enough that they can be easily replaced/rewritten. Anyways, i'm going off on a tangent here about the greater implications of using EOL stuff long term, but those are my opinions and i simultaneously do admit that there are exceptions to that approach and circumstances vary, of course.