This is the best site I could find for Luna/Terra stats:
https://terra.smartstake.io/history/3650

These charts are from half-way down the page:

There's some red in May 2021 where the UST supply decreased but otherwise mostly green.

I think supply has mainly been going up as:
(a) Anchor has one of the best stablecoin yields in the market (20%) which attracts a lot of capital, particularly now funding rates (and thus lending rates) have dropped.
(b) UST is expanding rapidly into other protocols. There's 100m UST on Osmosis now for example.
(c) Terra team swapped the community fund from Luna to UST causing that big jump in November.

However at some point that expansion of UST will stop and it will be interesting to see how Luna moves if the UST peg comes under pressure (like it did yesterday a little).

Interestingly another chart from that site suggests that supply charts for other stablecoins also have been going up only, at least over the bull market period. Except DAI which from memory had problems at one point:

But they remain a fairly small percentage of the overall 1-3 trillion crypto market cap in any case.

Which in turn does make me think about how exchanges pay people cashing out of crypto. From money coming in, rather than redemption of stablecoins, I suppose.

It would be interesting to know how the fiat reserves (and incoming fiat cashflows) of exchanges compare to the value of the crypto they are holding for users. At some point in a maturing market the fiat outflows will be larger than the inflows, and stablecoin supplies will start to draw down. May be something to watch out for.