Here is a short chronological summary of what the developers have said about this YTTV issue
2023-08-07 YouTube TV: Cable provider authentication failed - #11 by eric
2023-08-16 YouTube TV: Cable provider authentication failed - #27 by eric
2023-09-19 YouTubeTV TVE failing: This browser or app may not be secure - #3 by tmm1
2023-10-01 YouTubeTV TVE failing: This browser or app may not be secure - #18 by chDVRuser
2023-12-08 YouTubeTV TVE failing: This browser or app may not be secure - #82 by eric