Google Podcasts, the app (and website?) is shutting down. It will no longer exist. Re-framing it as its UI moving somewhere else is a bit like if WhatsApp was to be shuttered and moved to Facebook Messenger, and you said "it's not becoming unavailable, its UI is being moved to to Facebook Messenger".
Except for some exclusive podcasts, which Google has zero, podcasts are an open standard. Google Podcasts is just one of many clients, so there was never a worry of service availability. All podcast apps except Spotify are just different UI.
Podcasts are already in YTM though. Nothing’s moving. They’re killing off a product, plain and simple. Whether that’s no big deal or a big deal is up to every individual. I am assuming anybody who wanted to use YTM for podcasts already is, and it’s not a stretch to say people will probably be annoyed.