I note that in a git wiki it is said that this adapter works with Linux and is supported by OpeNWRT. I note also though that the manufacturers do not outright specify support for linux. I'm running the latest openwrt( not snapshot) on a Pi4b and just need an external wifi adapter to supply 2.4Ghz ap mode to older devices.
Would this suffice? and any recommended hardware should I look at? Note that price is a factor < $15 and performance isn't.
This thread might help - USB Wi-Fi that work in OpenWrt. Please add to list.
Another deep link to the page you were previously given: https://github.com/morrownr/USB-WiFi/blob/main/home/USB_WiFi_Adapters_that_are_supported_with_Linux_in-kernel_drivers.md.
The deep link page is where I got the wifi adapter from. I was just a bit confused about the review on it on the wiki stating it worked on linux when manufacturers do not mention it. Also not sure if it implies RPi4b
I'm guessing its safe to say it is RPi4 compatilbe. The alternative maybe a bit too steep for my liking
manufacturers don't care, and/or are clueless.
it works on all device where there's an USB port capable to deliver enough power to run the adapter.
Is there a way to tell that a wifi adapter is AP mode capable other than explicitly indicating that it is?
you can't even be sure you're getting the radio chip the seller advertises 
best references are probably the two URLs I provided.
The better solution than buying any USB WLAN card, would be looking for a cheap OpenWrt wifi-router to offload the wireless tasks in bridged-AP mode. Cheaper, faster, more reliable, more features.