there are several devices that are similar to this, named "T2U" "T3U" "T3U PRO", they seem to share a common chip (AC600), will they all work under linux-libre?

AC600 does not refer to the chip but to the capabilities of the wifi device. See an explanation here: https://en.wikipedia.org/wiki/IEEE_802.11ac-2013#Data_rates_and_speed The chipset for this model, Archer T2U Plus, is Realtek RTL8812AU (but see note below). If I remember correctly, some or all of those other models (T2U, T3U, T3U Pro) use other chipsets. For example, at least one version of the T3U Pro uses RTL8812BU rather than RTL8812AU. I doubt T3U Pro will work successfully with the RTL8812AU driver. Frustratingly, it seems like even products branded as the same model may be using different chipsets internally. I found one review of the Archer T2U Plus that says it uses RTL8811AU, even though the one I bought definitely has RTL8812AU. I'll update the main article to note this.

I found Aircrack-ng have made driver not only for rtl8812/21au, but also for rtl8814au. I compiled and installed it through DKMS and it worked perfectly on linux-libre 5.10.3 (on a recent version of PureOS 10). But I'm not sure if there's any blob inside Aircrack's source code so I can't just add it here... I'm not sure what should I do to identify a driver as blob-free, should I search for .cab or .bin files?

In this case, a huge array in the hal8812a_fw.c file.