Proof? The news I recently saw, if you're referring to that, actually said they'd offer a higher-bitrate 1080p option for YouTube Premium, not that they'd lock the (then) current bitrate behind YouTube Premium and downgrade everyone else.
Edit: I did some searching. In addition to that, the only relevant news I could find were about Google testing locking the 4K (2160p) resolution behind the Premium doors, which they ceased doing.
They did lower the bitrate back in 2020, IIRC, because there was so much more youtube being watched all of a sudden and they didn't want ISPs to be crippled under the weight. Don't know if they reverted that change since.
Edit: I did some searching. In addition to that, the only relevant news I could find were about Google testing locking the 4K (2160p) resolution behind the Premium doors, which they ceased doing.