According to the timestamp, Apr 2019, on extended-title-remover.php file in the download, the plugin is approaching two years without update. The last updated time in the plugin details appears to refer to the readme file.

Site details included the unmodified TwentyTwentyOne WordPress theme, WP 5.6.1 Multisite, running on Linux PHP 8.0.1.

Extended Title Remover appears in admin plugin list, and appears in WP editor with its checkbox. The box toggles and keeps its setting across editing.

But, the titles of both pages or posts remain visible, whether set individually or globally. Further, the failure is absolutely silent; no notice/warning/error emitted.

Suspect in function wpetr_supress_title() that array_key_exists(get_post_type($post_id),$extended_title_remover) was always returning false cuz array $extended_title_remover was always 0 length.

Good hunting

i guess this not work with my theme.