1---I have seen a lot of people forced to relocate.
This means "I have seen a lot of people who were forced to relocate." "Forced to relocate" is a past participle clause modifying "people."
2---I have seen a lot of people were forced to relocate.

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Although sentences of this type will certainly be found, they are not good English. The addition of "were" requires the addition of another word, and which one will vary the meaning of the sentence.
2a---I have seen
that a lot of people were forced to relocate.
This emphasizes the speaker/writer's knowledge of the fact that many people were compelled to change their residence.
2b---I have seen a lot of people
who were forced to relocate.
This is the same as sentence 1. The author has seen the
people, rather than the act or consequence of relocation, as in sentence 2a.