In trading card games, you can take a cheap/common card and use it in place of a more expensive or harder to get card. This can range from writing the name on it, to re-backing an unofficial print, as long as it's obvious that it is a proxy.

I'm not sure what the 40k equivalent terminology is, but it does have a rule that only allows authentic GW miniatures in official games. This rule is not just to keep GW's profits but to also prevent cheating as the game relies on the actual physical measurements of the miniatures.

As long as all players are fine with using proxies, and the rules don't prevent it, they're allowed to be used, but anything involving the IP owners is usually restricted to authentic items. In short, yes, it puts official tournaments behind a massive paywall, especially if a card/mini is no longer being manufactured.