What Happens to Blocks if I Disable Advanced Gutenberg?
Here is an example of a Columns Manager block, after the plugin was disabled.
If you click on the “Keep as HTML” button, you will see that the block uses normal HTML code. You can add your own CSS styles to your theme and keep on using this block.
Here's another example using the Images Slider block:
And here is the HTML code for that same Images Slider block: