Welcome to the forum. Episodes labelled leaving today are due to leave NOW TV at the end of the day, due to their rights having expired. Boxsets come and go on a regular basis and catch-up content usually expires 30 days after broadcast. Episodes often return in the future but it's a case of waiting for them to come around again.
However sounds like you are looking at SEason 5 where the last episode is due to leave today. However there are S1-4 still available for you to watch in the Nicktoons section of the Kids Pass, giving you 100-odd episodes of TMNT to watch if you haven't already.
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