As discussed in #25781 Symfony's Form and Validator components are pretty much hardcoded all over to support only 4 digit years. This made sense when they were written as PHP had no support whatsoever for large dates, and right now it's still flaky at best. It is however practically feasible today, likely since PHP 7, to provide fundamental support for large dates.

The so-called Year 10000 problem may seem highly hypothetical, but for example in scientific applications calculating nuclear waste effects it has practical consequences, and people may be building those in Symfony. It does also note that ISO-8601 doesn't bother with the problem and just states that years at 4 digits unless otherwise specified by data exchanging parties.

The RFC is to discuss whether we should even bother right now as Symfony, and research practical issues. What does it mean for data exchange, for example does (de)serialization support it if ISO-8601 doesn't? Do Doctrine's supported data backends support it? Etc.