I was doing some work in a column set and noticed validation errored when I tried to use @EndPeriod in a caption column. Running the report, it still generated correctly using @EndPeriod. I noticed on Row Sets, you can still select report parameters from the Formula window.
DPL standard Column Set from Acumatica:

DPL standard Row Set from Acumatica:

We just upgraded to 2026R1 but I noticed the same in a 2025R2 environment with the modern UI. Switching the column set back to Classic UI, the parameters validated correctly.