Er... so you end up with an 8- or 9-sprocket cassette with spacing that's incompatible with your existing 7-speed shifters and 7-speed chain? This does not sound like a particularly useful thing to do. Much better to buy a 7-speed cassette - which, as colin54 points out, are still available, Brexit/Covid/Suez problems allowing) and use spacers.
To the OP: the Flux comes with the 1.85mm spacer to convert its freehub from 11-speed to 8/9/10-speed compatible. To use the 7-speed cassette you'll need (I think) another 4.5mm of cassette spacers. You may find that your rear derailleur needs adjusting though. If you need a new 7-speed cassette, perfectly decent models are also still produced by SRAM and Sunrace, and overpriced premium quality ones by IRD. Good luck finding one in stock anywhere at the moment though.
7-speed is obsolescent in the sense that it's rarely spec'd on new bikes, and those new bikes that it does appear on are invariably cheap'n-cheerful* sub-£200 "mountain bikes". As the equipment manufacturers make most of their money selling to the bike manufacturers they are going to focus on what the bike companies want, so there will be less choice available in older standards like 7-speed. Similarly if your local bike shop mainly sells and services high-end bikes, it's not going to bother keeping many 7-speed cassettes in stock - unlike, say, a community bike workshop, which will always try to have a few on hand. That's not the same as obsolete, though.
* I say "cheerful" - it's all fun and games until the cheapo v-brake pulls apart at a crucial moment...