Graham --

I noticed this and I think it's a very clever fix to a problem that has given me all sorts of headaches over the years. I did a bit of poking around to "lightly" confirm my hypothesis... They've added the Apply button for views that have particular dependencies where instant filtering could cause unexpected/undesired results.

Ex: The Apply button appears when the view you are filtering is associated with an automation trigger. (There may be other situations where it appears, but this one I confirmed.) I created a dummy automation on a view and got the apply button in the filter -- note the warning at the top: 

So if you have an automation that sends an email to someone every time a record enters a view, live updated filters can cause a zillion emails to be sent without you intending as you make edits to filter conditions. (Will admit I've made this mistake...) This prevents that problem. I bet there are other situations where the filter button appears as well, but I haven't dug too deep. 

Oh interesting! Hmm.... I will have a look on it. 

Might be that an automation we made in a particular field is affected. I will look into this. Thank you !