Technically you can set the base breakpoint to whichever one you choose. There are some considerations and best practices.
The reason “lg” is usually chosen as the desktop base breakpoint is because it reflects the most commonly used desktop resolution and therefore you can start there and work outwards in both directions.
With mobile-first it’s usually very similar on the smallest sizes until about “md”, so it makes sense to have the base as the first (smallest) breakpoint.
There is no right or wrong approach and there are many different opinions and reasoning for which is better, the most important thing is that it works for you.
One advantage I find with mobile-first from a development perspective is that on mobile resolutions blocks are usually stacked in a lot of cases, which is the default, so it can lead to less overridden styles.
One disadvantage is that clients and non-technical editors may find it unintuitive if they are used to looking and thinking about it from a desktop perspective.
Exactly, this is precisely the reason I proposed this feature, because then clients don’t need to think about which breakpoint they are on when editing.