This is probably more feasible than the proposal in bpo-41254 since it's a well-defined spec (mostly — it includes an optional alternative format and the number of digits allowed is defined "by agreement", thus defeating the purpose of using a spec in the first place) that's not even particularly difficult to implement, but there are still a few problems (and one reason I've never implemented this, despite desperately wanting a better string representation for time deltas). Two minor problems first:

1. Unlike ISO 8601 datetimes, these are not especially "human-friendly" formats, so I don't think they're especially useful for displaying timedeltas.

2. Also unlike ISO 8601 datetimes, I don't think these are in particularly wide use, or widely supported. That's not a major strike against it, but if it's not useful as something to show to humans and it's not especially useful as something to show to / read from other computers, that weighs against its inclusion in the standard library.

The biggest problem, however, is that `timedelta` does not and cannot represent "Year" or "Month", which means that `P1Y` or `P1M` would always need to be invalid to parse. We could eliminate this format, but it means that we would never at any point in the future be able to implement a parser for the full spec. Since the concept of a year and a month are ambiguous and at least the 2016 version of ISO 8601 doesn't seem to define what it means for a duration to last 1 year or 1 month, you can't even really count on such a thing as an interchange format, because different implementations might give you different results! What does `20200131T00:00:00/P1M` represent? The interval (2020-01-31, 2020-02-29)? (2020-01-31, 2020-03-02)? Something else?

A better target for parsing ISO 8601 durations would be something like `dateutil.relativedelta`, which does have defined semantics for years and months (though as I mentioned above, those are not necessarily consistent with the semantics of other libraries parsing or writing out this format).

I am also not entirely clear on whether "weeks" is just an alias for "7 days" or if it means something related to weeks in the ISO calendar (and if that makes a difference for durations).

I imagine that generating these formats is a bit more forgiving, because you would simply never generate the forbidden formats, and we can offer configuration options in the formatter method to allow the user to tweak the various ambiguities in the spec.