Would something like the umarex legends be prohibited under c21?
Yes, it appears that the Umarex Legend series of replica airguns would be prohibited under C21 if it becomes law. If you want to own one then now is the time to buy. IF the law comes into enforcement then apparently you can still own and enjoy but cannot sell, give away, trade or buy more. Many non-PAL airguns could become prohibited if a PAL version exists such as the Crosman 1377, Crosman 2289, Crosman Optimus and all the airguns detuned for the Canadian non-PAL market. They all exactly resemble the full powered version of the airguns (firearms). Usually the RPAL version of an airgun is not available in Canada. But the detuned non-PAL still exactly resembles an RPAL firearm version.
The definition replica firearm in subsection 84(1) of the Act is replaced by the following:
replica firearm means any device that is designed or intended to exactly resemble, or to resemble with near precision, a firearm that is designed or adapted to discharge a shot, bullet or other projectile at a muzzle velocity exceeding 152.#4 m per second and at a muzzle energy exceeding 5.#7 Joules, and that itself is not a firearm, but does not include any such device that is designed or intended to exactly resemble, or to resemble with near precision, an antique firearm; (réplique).
https://parl.ca/DocumentViewer/en/44-1/bill/C-21/third-reading
It appears that the intention is to allow you to continue ownership but no further transfers or acquisitions. We will have to see how this part plays out.
https://www.ofah.org/firearms/bill-c-21/
The other replies to your question explain how the current law applies to the Umarex Legend series of airguns.