According to Halacha, is the prohibition of leaving techumin on Shabbat biblical or rabbinic in origin? Not that it wouldn’t be issur if rabbinic, but it would have implications for Kavod Habriot and other things.
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Most common opinion is that 1 "mil" (let's assume 2/3 a mile) is rabbinic; 12 "mil" (let's say 8 miles out) is Biblical. It's not so simple to just hand-wave and say "oh if rabbinic then I can decide it doesn't apply to me because I feel [X]."
The mishna says if someone did somehow wind up outside the techum without permission to do so, they are supposed to stay pretty much where they are for the rest of Shabbos -- but can walk away from that spot to relieve themselves.
There's the famous question of whether to allow the emergency responder to return home after taking an emergency call on Shabbos, as otherwise s/he may be reluctant to take the next call. The mishna in Rosh Hashana says that those who (correctly) broke techum to report the new moon and did so were then constrained to one big building, but the rabbis realized that would make them miserable enough that they might not come again; instead, the rabbis allowed them to walk all over Jerusalem.
At first glance, that means they're breaking rabbinic techum, which would mean that we'd let the emergency responder take a taxi home, but not drive their own gas-powered car. However Rabbi Shlomo Zalman Auerbach (master of the thought experiment) noted that some of these "new moon" witnesses may in fact have arrived from 11.9 "mil" away; the "let them walk all over Jerusalem" would have them crossing the Biblically-prohibited techum line. And yet that was also allowed because "otherwise they might not come next time."