Actually, all there is evidence for is that tp makes different things obligatory, or -- better -- leaves different things optional. When the need to introduce a temporal or aspectual or even modal distinction has arisen, tp has come up with the devices needed, whether accidentally or officially (the 'la' fragments seem to have always been there, 'luka' was an early accident and heartily condemned but has survived). In any case, tp already does have restrictive relatives in place. just not as clearly marked nor as differentiated from nonrestrictive (which is often a contextual matter even in English -- do we need and intend the information as part of the identification of the object): replace the 'pu's here with periods and the 'ona's with repetitions of the 'x ni' expression ('ona' is officially always backward looking, anaphoric; cataphoric, forward looking, use is rare, if occurring at all, but the only non-standard use actually commented on is the indefinite third person "one, they", which is, ironically the etymon of the form). (Rhetoric would advise putting the clause first.)
The oldest comments on 'pu' call it, in French, a virgule, an amorphous term covering several comma like punctuations, including ":", the only one to actually occur in official tp, though commas have been used for clarification.