from the clues so far in the Canon, the T-65 X-wing debuted fairly close to the rise of the Empire. (the U-wing post-dates it, per the visual dictionary supplementary material (which was written by the development team), and the U-wing is also said to be the last design Incom created before the Empire nationalized it.. which is indicated to have happened shortly after the Empire took over the Republic. we've had verification that the U-wing is an old craft too in rebels, where we are told it predated the Empire's standardization of hardware interfaces (which is why fitting the TIE Defender's hyperdrive to Ryder Azati's U-wing was not a sure thing.) we've also had supplimental material that indicates the X-wing was being sold to planetary militiai's prior to the nationalization.
so if there is a story about the Rebellion getting the X-wing, it won't be the old Legends one of stealing advanced protoypes a few years before ANH, it'll be about some Rebel operatives in 5bby - 10bby arranging a black market deal for some decade old ships.
or an infltration into the nationalized Incom offices to steal the no-longer-in-use plans so that rebel sympathetic companies and workshops can start building new builds for the Rebellion in secret.
