Practically speaking, if their phone can't handle 32K of jQuery, it probably can't handle Google or any other search engine they'd be using to find the site in the first place.
Gov.UK is slick af on Jelly bean Chrome 85 sideload with dozens of lengthy pages in active reference. Upgrading unnecessary. In the new economy any clever MVNO could turn L3 rules into subscriber market lead by keeping orphan handsets usable in this way.
https://www.obamaphone.com/