HTTP/3 has just been finalized as a standard this year, so I'd guess we'll start to see it be adopted maybe in 2028 or so

It will be easy to add support on the back end (at least for any software that already supports HTTP/2). So adoption will be limited by the front end. Adoption may come sooner, but only if telcos and phone makers both agree that it will help them. (Mobile currently represents ~75% of web usage, and still growing.)