Thank you for explaining your reasoning!
I’m adding one star for considering the interests of the overall ecosystem (although supporting newer versions doesn’t mean not supporting older ones).
I would leave forcing the PHP version up to WordPress, if I were you (which I’m not, of course). You point out that the existing users of your plugin would just not get updates past 1.9.3, however you’re missing out on new users. If I go to “Add plugin” and search for “notifications”, I get 5043 plugins listed. Your plugin comes up in the 8th place, however it is the only one on the first page that is marked as incompatible with my installation. On the first three pages, among 108 plugins, there are just 2 more like that.
As per this article – https://peq42.com/blog/wordpress-to-finally-start-dropping-old-versions-of-php/ – about 50% of WordPress installations are still using PHP 7 (April 2024). Which means you’re needlessly (unless your plugin actually makes good use of the new PHP 8+ language features) losing out on half of potential new users for your plugin.
BTW, where has WordPress, as a project, announced that PHP 7.4 will be dropped within the next 8-months? The latest WordPress is supporting even 7.2. It seems unlikely that they’ll drop support for 7.2., 7.3, and 7.4 just after 8 months now.