include suppression lists in segments?

Depends how your using/reporting with them.
I tend to remove all dead leads i.e. in to active and non-active in the last 12 months but every business id diferent.

I would say that it depends on the segment and its purpose. You might have a lead that unsubscirbed subscribe again (unlikely, but still possible). Is there a reason you are concerned with including them?

Sherin,

In general, it's cleaner for reporting purposes to exclude dead leads. You'll get a more accurate indication of deliverability, open, and click rates, which are important to keep track of for your org.