Getting autoconf 2.70 into Debian would be nice, and your willingness to help is great. You might want to have a look at
https://bugs.debian.org/977331 which is the ITA bug for autoconf, and discusses both finding a new maintainer (Matthias Klose <d...@debian.org> looks to be considering taking it on, but I imagine he'd welcome assistance; such a critical package as autoconf really should have a maintenance team) and what getting 2.70 into usage would require. It seems the plan is to generate a draft package 2.70 and then rebuild everything with it, to see how much breaks and how, and use that information to help decide on a concrete plan. They seem pessimistic about getting it in as the default in bullseye, which seems like a shame. --Barak.
