Wanted to spy on my dog, ended up spying on TP-Link

Are techniques like using Frida and mitmproxy on Android apps still going to be possible after the signing requirement goes into effect next year?

Overall: yes, but it will get much harder for apps which need attestation, which is sort of the point, for better or for worse. As far as I know you'll still be able to OEM unlock and root phones where it's always been allowed, like Pixels, but then they'll be marked as unlocked so they'll fail Google attestation. You should also be able to still take an app, unpack it, inject Frida, and sideload it using your _own_ developer account (kind of like you can do on iOS today), but it will also fail attestation and is vulnerable to anti-tampering / anti-debugging code at the application level.

So for people with any practical needs what so ever (like banking): No.

At this point Android isn’t meaningfully an open-source platform any more and it haven’t been for years.

On the somewhat refreshing side, they are no longer being dishonest about it.

Open source has nothing to do with hackability.

Firmware which requires updates to be signed with a manufacturer key can still be open source. As long as its code is available publicly, under a license which lets the user create derivative works, it meets the definition. You can still make a version of it that doesn't contain that check, you just can't install that version on the device you bought from the original firmware developer. Some FIDO keys (and I think Bitcoin wallets) do this.