Late in cancelling National Health Insurance

Hello,

I studied in Japan for 7 months in 2018 and was enrolled in the National Health Insurance programme. I remember going to my Town Hall as I was preparing to leave Japan and I was given an envelope that I was supposed to send my National Health Insurance card in to cancel my NHI. Checking my old wallet for an upcoming Japan trip I now realize I never sent it in since I have my NHI card.

I have been trying to look for information on how to solve this but I don't know what repercussions there might be for having completely forgot to cancel the NHI during the last 4 years since I left Japan.

Can I send it now? Or can I go to my old Town Hall when I travel to Japan this upcoming spring? Or will I be unable to enter Japan due to this?

Please, any help is appreciated!

did you write a form for cancellation or not ?
if you wrote, you already canceled it.
if not, you did not cancel it.

just show an example,
you go to city office to cancel the insurance on Nov. 9. but, you will leave Japan on Nov. 15. in that case, you write the form for cancellation on Nov. 9 and they give you an empty envelope for sending back your insurance card to city office, because it can be valid until the date you wrote as your postulated(you wrote in the form) leaving date. (i.e. the card is valid until Nov. 13.) you was expected that you post it in mailbox until you left Japan.

returning the card is not enough to cancel the insurance. you need to write some document.
so, because you have an envelope, I expect you did necessary paperwork for cancelation.