For the Canon EOS 1300D, you can see the calculated DLA of f/6.8 at that camera's review at The-Digital-Picture. Bryan also has a short article about what DLA is and how it affects images.
All of the Canon cameras reviewed at The-Digital-Picture since about 2004 or so have the calculated DLA listed. It's included in the "Specifications" list for each camera. There's also a chart in the main body of the review that will show DLA along with other sensor specifications and compare them to other similar cameras. The list for the Rebel T6/1300D is fairly brief. If one looks at the list included in the review for, say, the EOS 5D Mark III, there are a lot more sensors with varying pixel sizes in the comparative list.
Since DLA for a digital sensor is strictly a function of the size of its photosites (a/k/a pixel wells), any other camera with the same pixel pitch should have the same DLA. Even if you are interested in the DLA of a non-Canon sensor, as long as the camera in question has a Bayer mask in front of the sensor it will have the same DLA as a Canon camera with the same pixel pitch.
With regard to specific lenses: Lenses do not have a DLA. But if a lens is "soft" enough, even at its maximum sharpness, it may still project airy discs that are larger than the DLA of a camera to which it is attached. In which case the DLA of the sensor would not be the most restrictive impediment to getting the sharpest images possible. Rather, it would be the resolution limits of the lens.
Particularly for the EF 75-300mm f/4-5.6 III (or any of its predecessors), there's a very real possibility that the lens will not be able to resolve points to sizes as small as many cameras' pixel size. You'll find the EF 70-300mm f/4-5.6 IS STM or the EF-S 55-250mm f/4-5.56 STM lenses noticeably sharper than the EF 75-300mm f/4-5.6 III.
What we call "sharpness" is really a combination of many variables. Among them are contrast, resolution (in terms of lines per millimeter or lines per image height that the lens can project), diffraction, chromatic aberration, astigmatism, etc. That's before we even get into manufacturing tolerances and variation from one copy of a lens or camera to the next.
Ultimately each specific lens and each specific camera used together can have slightly, or sometimes more than just slightly, better or worse optical performance that contributes to what we call "sharpness." Tests done by review sites often are done on a single copy of a lens or with a single combination of lens and camera body. One good source for lens data that usually includes at least ten copies averaged together is Roger Cicala's blog at lensrentals.com.
At the very least, you should look at tests of lenses that were done on the same or similar cameras to the one(s) you intend to use with a prospective lens.