Disabling Full Site Editor (FSE)

No, because the site editor is turned off by default. Unless you’re building a block theme, or declare that your theme supports block templates in PHP/theme.json, it doesn’t apply to you.

WP version 6.2 (almost released) has many new features, including a more enhanced Full Site Editor (FSE).

This is not true, 6.2 removes the beta tag on a feature that has been in WP core for several major versions, and powers the last 2 default themes.

For instance, suppose that I as a developer have built a very customized theme for a client. It would appear that with FSE, the client – or a co-admin – without proper authority from the site owner, could make little or big changes to the entire site, breaking the design that the client wants.

FSE is for block themes, not classic themes, the site editor cannot be used to modify PHP templates.

But more importantly, blocks can be locked, either to prevent their removal, or prevent their movement. This is not a new feature.

Likewise, blocks can be constrained via theme.json, e.g. you can limit the colour options to a set palette, and you can do it on individual blocks. You can disable options entirely. For example on my current project I’ve allowed group blocks to have backgrounds in 4 preset colours, but users cannot set their own, change text/link colours, or access font controls. I’ve also disabled most layout controls, forced spacing to 3 values, and eliminated all letter spacing and line height controls. Most of these are opt in and have shortcut parameters to turn them all off by setting false on a value.

Of course, the client is in charge of their site, but well-meaning changes (or just messing around) could screw up the entire site.

This is possible in a classic theme, with all the same solutions.

However, assuming you made changes to enable the site editor, and did not do your due diligence in enabling/configuring theme.json, and your client got themselves in a huge mess customising things:

  1. their “mess” would be in a post of type wp_template or wp_template_part, and your theme would be untouched, the site editor does not edit files.
  2. Revisions!!
  3. The site editors UI has a clear customizations button, allowing them to delete the template posts and reset back to your themes defaults
  4. Backups

But most importantly, you didn’t build a block theme so this is all theoretical anyway.