Greetings,
This is Tin, I understand that you want to delete a plan in the web version of MS Planner.
To do that, please go to https://planner.cloud.microsoft and sign in your MS 365 account, please choose the plan that you want to delete, please click the 3 dots in the top right corner and select Plan Settings. Please scroll down and look for the Delete plan option.
Note: Please be reminded that the plan owner is the only person who have capability to delete a plan. Please also remember that it will remove the linked Microsoft 365 Group and all its content. If you just don't like to see the plan, you can just Leave the plan instead of deleting it.
Thank you,
Tin