In the first sentence "that" can be considered as a demonstrative pronoun and "which" as a relative pronoun. Of course, you can't take both "that" and "which" to be relative pronouns as only one can appear in a relative clause; two makes no sense (Lexico).
- When he travelled away from home, he was separated from all that is familiar.
or
- When he travelled away from home, he was separated from all which is familiar.
In these preceding two grammatically correct sentences, "all" is an indefinite pronoun (not the case in the combination "all that which", though; considered next).
In the combination "all that which" as used in the sentences in the context of this question, "all" is an indefinite determiner determining the demonstrative "that"; "all that" is a noun phrase with the pronoun "that" for head (pronouns as heads).
- All is forgiven.
- All have arrived.
The first possibility (demonstrative "that" followed by relative "which") is a combination that is used less nowadays (ngram), but which is apparently irreplaceable. In the next sentence, for instance, the suppression of "that" seems to reduce the force of the assertion.
- (ref.) or are only those things which we see, or in some way perceive through the bodily organs, truly existent, and no others besides them? And is all that which we call an intelligible essence nothing at all, and only a word?
Besides the fact that both constructions inquired about are correct, both can be reproached with a lack in specificity which may leave the reader perplexed; the tense, as remarked in user Xanne's comment should preferably be the past, which would give to the expression the specificity it lacks. The use of the state present (is) is however possible, provided you add a modifier to the adjective. An example is shown below.
- When he travelled away from home, he was separated from all which is familiar to a boy of his age.
