"Doesn't have much market share" really depends I guess on your domain doesn't it? For example, if you're building something that's going to get a fair bit of mobile traffic iOS Safari would represent a fair chunk.

Besides, most applications of backdrop-filter would fit into 'progressive enhancement' and the rest of the browsers that don't support it would just fall back no a non-blurred transparent background which is fine most of the time.

Yup. I noticed Apple has updated the top navbar on the new product pages where it is enabled. It falls back to unblurred-translucent container on other browsers.

Hopefully other browsers will catch up soon.