In my bubble, there's no alternative right now. Switching to a more modern browser requires an enormous amount of agility throughout the pipeline that does not exist. To adopt a shiny new browser, agencies would need to push browser updates to all their client machines every ~4-6 weeks [2]. Solution providers need to perform continuous testing on their web apps (browser vendors do not guarantee minor updates are backwards compatible), creating patches, which need to be submitted to the government for regression testing... and oops that took too long and the next EOL is here.
With Microsoft's surprise obsolescence of Internet Explorer 11 across all its cloud offerings 1Q 2021, we're headed straight into a brick wall. Very interested to see what shakes out here.
I suspect initially we'll see the government just deploy and use older unsupported copies of Chrome/Edge browser, under the guise of "well it's better than IE11 right?". Longer term, ...?
[1] https://blog.tailwindcss.com/tailwindcss-v2#incompatibility-...
[2] Vendors do not support these versions after EOL for any reason. Firefox has an ESR branch, bless them, but some agencies have now increased scrutiny of Mozilla software for recent political/security reasons.