In the modern form editor, when adding a subgrid component, you are by default presented with the option to select any table. To only show tables that have an actual relationship to the current form's table, you need to tick the box "show related records". In practice you ALWAYS need to select this.
Ever since 2011 when the subgrid feature was introduced, I don't think I've ever ran into a requirement to show records from another table on the form that have absolutely no connection with the current record. I believe ~99% of app makers are therefore required to now remember to tick the filter box, so that they don't need to browse through all the thousand tables within the environment.
Besides, since there is now a table named "[Deprecated] Dynamics Customer Service Analytics" in every environment, it is alphabetically sorted to be the first thign every app makers sees in the subgrid configuration experience. All of this could be easily hidden if the "show related records" filter would be on by default.