Do we assume two SNPs further than 500Kbp from each other are in LE？
They are not necessarily in LE, but it is true that at increasing distance LD decays, and if distance is great LD tends to be very low.
In addition, I think there were computing time issues there. LD is computed between all possible pairs of SNPs. If in a chromosome you have 10000 SNPs, you need to perform 100009999/2 4.99510^8 comparisons. If only 1000 SNPs are closer than 500kb you "only" need 1000999/2, i.e. 4.99510^6 comparisons.
Given that SNPs far from one another are quite a lot and that they are usually not in LD, here you havve the motivation!
found the official explanation
And there is indeed a database for long range LD
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