We both assume and expect that non-default browser users are keeping up to date. If someone is ten releases out of date on Firefox and something doesn’t work we advise them to update their browser. Wipe hands on pants.
For IE we’re a little more forgiving, but will still stay within the most recent - 1. This is mostly due to environments where users are forced by IT to only use vetted software and they are locked into IE.
What we’ve seen is that people skip using IE and go straight to the mobile apps if they are hamstrung by IT. IE makes up less than 10% of our user base, all versions considered, and we’re in the construction industry where users are typically under trained and less capable with tech.