To go offline in slack should be an option. Talk to your manager or someone at the company. You have a disability, there must be accommodations.
What I do is keep any chat and such closed and only accept e-mail. I then have two set times a day where I check it by going through the mailbox. There should be no argument that you can't do this unless you're employed to be always on-call and need to be responsive in 5 minutes.
Depends on the role and company. A big company can probably tolerate a couple non-team-player developers who just execute on tasks they're given in isolation, but most of the time software engineering is a team activity and being unresponsive to teammates is going to block them from getting their work done.
I'd rather have one developer take a hit to their productivity to answer a coworkers questions and keep the whole team moving forward than have the team blocked so that one employee can complete their ticket in a single uninterrupted block.
Being offline doesn't help with distractions. People will send messages anyway and that red dot will be there. This is a long standing issue with Slack, you can't silence it completely. Even muting channels doesn't help, you will get a notification or the red dot anyway.
The only way is to go nuclear: uninstall it from your phone and close the desktop app. But that isn't doable in most companies.
What I do is keep any chat and such closed and only accept e-mail. I then have two set times a day where I check it by going through the mailbox. There should be no argument that you can't do this unless you're employed to be always on-call and need to be responsive in 5 minutes.
Make sure to generate a paper trail when asking.