What the rule actually does
From this year, anyone whose Social Security wages from their employer last year were above the threshold must make catch-up contributions as Roth. The usual way this gets described is that the contribution “becomes Roth”, which makes it sound like a change of label. What the rule does is narrower and sharper than that: it takes away permission to make the contribution pre-tax. It does not grant permission to make it as Roth.
In a plan that offers both, those come to the same thing. In a plan that offers only pre-tax contributions, they do not. One option is removed and the other was never there, so there is nothing left. The catch-up cannot be made.
Why this catches people
Almost every explanation of this rule is written from inside a plan that has a Roth option, because most large plans do. From there it reads as a tax question. Pay now or pay later. From inside a plan without one it is not a tax question at all, but a hard stop on a contribution somebody was counting on.
The natural assumption in the gap is that the contribution falls back to pre-tax, the way most rules leave you where you started when an option is unavailable. This one does not, and nothing in the wording suggests otherwise once you look for it. The usual first sign is a payroll election that will not go through, often several months into the year.
What to do about it
The question worth asking your plan administrator is narrow, and it is not “does my plan offer Roth”. It is whether the plan accepts Roth catch-up contributions for this year specifically. Plans could add the feature for exactly this reason, and a plan that offered no Roth at all last year may well offer it now. A plan that has not added it cannot be worked around from your side.
Our Roth catch-up mandate calculator works out whether the rule reaches you at all, and what the blocked outcome costs if it does.
If the answer is no, the contribution is closed to you this year and the money has to go somewhere else. An IRA has its own limit and its own rules. A taxable account has neither. Both beat leaving the decision until December. If the answer is yes, the rule costs you a deduction rather than a contribution, and that is a cost worth pricing rather than guessing at.
Either way it is worth checking early. The remedy for the first case is a plan change that takes time, and the remedy for the second is a payroll election you would rather make in January than in November.