Hi @steve_hole
I think the best (and maybe only) way to do this is to create a segment. Sadly you can't use RegEx in segment builder so it is a little handywork.
Use the "matches" operator to enable wildcard characters.
Add the browsers you want to focus on:
- IE 11 was the last one so you just include all of them
- Google Chrome 50.0
- Google Chrome 4* (will match 40.0-49.9)
- Google Chrome 3* and so on
- Safari 15.5*
- Safari 15.4* and so on
You can already use this segment in your workspace report on Visits or create a calculated metric with it.
- Add the segment to the calc builder and add Visits
- Devide by Visits
This is basically your "Unsupported browsers share"

I mean the way to setup the segment seems a little blunt to me so I would be interested if there is a more elegant way