As expected, all answers about The Doctor and regeneration tie back to The Timeless Child.
Steven Moffat seems to express here that evidence of The Doctor’s uncapped regenerations was shown in the Season 13 serial, “The Brain Of Morbius.”
The Doctor’s regeneration status was just one of many things CinemaBlend talked to Steven Moffat about.
And now that we have assurance from Steven Moffat that the show will stay the course with the Timeless Child storyline, one would think we’ll continue to see more about The Doctor and Ruby connecting through their shared status as foundlings.