I am not sure there is a language where the median user's code looks more different from 2012 to now than R's. RStudio and the tidyverse have basically created an all-new "standard" library for the language that has gotten extremely wide adoption, and it tends to smooth over a lot of R's warts if you stay with code that has been written to fit with the style of code that the tidyverse encourages.

This sounds like something that in most other languages, would have become R v2 (non-backwards-compatible)? Why does R retrofit instead of enforce best practices?

The short answer is that "tidyverse" is a set of third-party packages that was developed by a different team than base R. It's not a modification of the language itself. This is possible because of certain features of R that make it particularly syntactically flexible (see, e.g., https://adv-r.hadley.nz/metaprogramming.html).

I know of exactly nobody who was happy with how the Perl 5 to Perl 6 transition went, or the Python 2 to Python 3 transition. I think not ever having a firm cutover like that was beneficial to R.

EDIT: And yes, as citrate05 says, it's just an additional set of libraries. There's no changes to the language itself.

Python wasn’t pretty but I’m happy it was done. Than if we were still on python 2 with only a portion of the changes. There would be a lot of discontent if that was the case too.