The IRS released the unpublished version of the final catch up regulations which should be published in tomorrow's Federal Register.
For 94 pages of reading enjoyment for those who can wait, see here https://public-inspection.federalregister.gov/2025-17865.pdf
The final version should be here tomorrow https://www.federalregister.gov/public-inspection/2025-17865/catch-up-contributions
One interesting right up front is
"Applicability date: These regulations generally apply with respect to contributions
in taxable years beginning after December 31, 2026. However, see
§§1.401(k)-1(f)(5)(iii), 1.414(v)-1(i)(2), and 1.414(v)-2(e)(2) and the Applicability Dates
section later in this preamble for additional details regarding applicability dates."