Unfortunately the developers weren't able to get Google to work with them on a policy change that impacted the app [0] [1] and so gave up and have moved on to a new and completely different customer support service.
[0] https://developers.googleblog.com/2020/08/guidance-for-our-e... [1] https://mailplaneapp.com/blog/entry/mailplane_stopped_sellin...
So unfortunately
I use non-default Gmail UI settings. I'm still using the classic theme, dense settings, etc. I'll try again, because I'd really like to use Mailplane as long as it will survive.
I'm aware why Mailplane has been EOL'd. But the developer also claims he's trying to keep it alive as long as possible and he's released at least one version since that blog post, so I'd hoped he would be willing to release a universal build. I don't know anything about the internals of Mailplane but I guess it's a non-trivial amount of work to do so.
I've fallen back to using Fluid, an ancient and also Intel-specific SSB, but its web content runs in a separate WebKit ARM process so it's plenty fast.
I've emailed the Mailplane author but they wont release an Universal version of the app since they've EOL'd Mailplane.
I have yet to find a Gmail SSB that I'm happy with under ARM. Fluid is a barely workable solution.