How to revert back to legacy widgets style page.?!

Unfortunately it isn't possible at the moment - you would have to deactivate the entire Gutenberg plugin and rely on the version shipped in WP core. There's a plan to address this shortcoming in the nearest future. https://github.com/WordPress/gutenberg/issues/24949 is related. There's a PR here that makes it possible using the classic editor plugin: https://github.com/WordPress/classic-editor/pull/137
Wait :) There are two ways to get editor (screen) back:
add_filter( 'gutenberg_use_widgets_block_editor', '__return_false', 100 );
or you can also access the old screen via the old url: SITE_URL + /wp-admin/widgets.php
Neither of the above will bring back the old widgets panel in the customizer as long as the Gutenberg plugin is active. This is a bug and there is work in progress to disable that via the classic editor plugin.
Closing this one since there's nothing actionable here, and the response by @draganescu above provides solutions on how to get the old screens back :+1:
Most helpful comment
Wait :) There are two ways to get editor (screen) back:
or you can also access the old screen via the old url: SITE_URL +
/wp-admin/widgets.phpNeither of the above will bring back the old widgets panel in the customizer as long as the Gutenberg plugin is active. This is a bug and there is work in progress to disable that via the classic editor plugin.