Excel already has the ability to write custom functions in Javascript (and VBA), this is the ability to write them in the formula language rather than JavaScript.

(So no, Google Sheets does not support this)

The latest improvements in Excel really do seem to be re-widening the gap between Google Sheets and Excel (Dynamic array formulas, Let, Custom data types, powerquery improvements...)

Is it not, though? Google Sheets is available to all GSuite customers. GSuite is the most popular email hosting platform in the world, with roughly 18% market share (Microsoft 365 trails not significantly far behind).

I think Google Sheets has a pretty solid user base.

As already pointed out, these are not the same. Javascript based custom functions in Google Sheets are actually really error prone, meaning that they will just _randomly fail to execute_. Cells will be stuck with a "loading..." message until you trick Sheets into recalculating the value. In the next couple months, I'll likely be ripping out as many custom functions as I can.

And these are not for particularly complicated functions.

I'd really love to have this feature in Sheets. It would simplify a lot of what my sheets do, and also make it more accessible to a Sheets power user that gets scared off from code.