Casting Spells And Rest Interruption - How Does It Work

We were surprised to learn recently that casting a spell can and does interrupt any kind of rest in D&D 5e. In all games we've played thus far, our DMs have allowed us to perform "downtime" spells, specifically ritual spells. This is however, technically not allowed.

From the official rules - provided by Roll20, we can see that casting spells does interrupt a long rest

A long rest is a period of extended downtime, at least 8 hours long, during which a character sleeps or performs light activity: reading, talking, eating, or standing watch for no more than 2 hours. If the rest is interrupted by a period of strenuous activity—at least 1 hour of walking, fighting, casting spells, or similar adventuring activity—the characters must begin the rest again to gain any benefit from it.

While this doesn't specify a short rest too, you could argue that the same could be said for interruption of a short rest.

We're pretty casual with this one. We leave it down to the DM to decide. As an example, we had an artificer in one game that wanted to repair a cloak using the mending spell. Our DM decided that this would constitute as low effort activity, and therefore wouldn't interupt the rest.

However, when it comes to more powerful, impactful spells - the tendancy is to not allow them to be cast.