I had a similar experience. On a project I'm working we had to support IE11. Slate docs mentioned that it doesn't fully support IE11 out of the box, but that support can be achieved using polyfills. This turned out to be false and that part of documentation was very old and inaccurate unfortunately.
After few weeks of development we ran into IE11 issues we couldn't fix and Slate devs expressed that they are not at all interested in supporting IE11 in any way, which is a fair point of course since IE11 is on its way out and supporting it is a big chore. There were some PRs that were supposed to address some of the issues, but development is kinda all over the place on Slate so we weren't confident enough to go with it and decided to switch to ProseMirror.
It hasn't been easy and ProseMirror can be complicated, but we managed to work it out and created what we needed.
After few weeks of development we ran into IE11 issues we couldn't fix and Slate devs expressed that they are not at all interested in supporting IE11 in any way, which is a fair point of course since IE11 is on its way out and supporting it is a big chore. There were some PRs that were supposed to address some of the issues, but development is kinda all over the place on Slate so we weren't confident enough to go with it and decided to switch to ProseMirror.
It hasn't been easy and ProseMirror can be complicated, but we managed to work it out and created what we needed.