What effect did Zapdos-Galar have on the metagame?
Zapdos-Galar pretty much dictated the metagame around itself for a few months alongside Tapu Koko. To rival a Pokemon like Tapu Koko in impact, you have to be either insanely splashable or broken, and Galarian Zapdos was the latter. No Retreat sets dominated the metagame and had little counterplay, which restricted teambuilding heavily and even moreso dictated how games were played, as offensive pressure was pretty much the most reliable way of beating it.
In what main roles was Zapdos-Galar used?
No Retreat Life Orb sets were the strongest, and with Roost and Thunderous Kick it could outlast and wear down "checks".
What caused it to have a significant impact?
Being super broken meant that teams had to prep super hard for it and even then it could easily get past them. It ran No Retreat Dragon Ascent Thunderous Kick and Roost, and only a small handful of Pokemon could switch into that. It was primarily popular in 2021's STABmons Open and OMPL, and was banned near the end of that OMPL.
How do/did you deal with this Zapdos-Galar in STAB?
Offensive pressure from things like Tapu Koko and Tornadus-T before it could set up was the main way, while defensively you had to run something weird like Zapdos or try and live a hit with Toxapex and Haze it. You almost always lost at least one Pokemon to this threat, and often more.