The Wordfence Security plugin says the TwentyTwenty plugin required by The Landscaper theme is outdated by 4 years and unstable / unsafe to use.
The TwentyTwenty plugin has, indeed, not been updated for a long time. This plugin won't add any option pages, or heavy PHP scripts to the theme, which could be vulnerable. It is a simple shortcode plugin that turn 2 images into a before/after slider, most likely the plugin won't break anything.
Of course you're free to remove this plugin from the theme, as this plugin is not necessary for the theme to work
If you want you can use this one instead: https://wordpress.org/plugins/before-after-image-slider/ which has been updates 4 months ago.
If you have any questions, please let me know.