In the Presentation tab of Structured Text fields, it is now possible to enable only specific heading levels:

This will have the effect of hiding the relative slash commands on the editor (in this case, the Heading 1 option is not present):

For greater convenience, markdown shortcuts for heading levels that are not available will fall back to the nearest available level — that is, if Heading 1 is not available, pressing # will insert a Heading 2 — and tab and shift+tab keyboard shortcuts, which have the effect of decreasing/increasing the heading level, will skip the levels that are not available: