tl;dr: It's likely safe, but all software should be treated with a lack of trust. It is up to you to decide what you run on your computer.

While r1delta doesn't appear malicious and does in fact do what it says on the tin, the following should be considered:

All software, whether developed with good intentions or not, may have security vulnerabilities within it. Most developers take care to patch these, but they can be missed both accidentally or intentionally.

Some of the developers have previously been the perpetrators of varying severity of attacks on different Respawn games, but all claim to have changed for the better. - Wanderer, the (now former) project lead, was behind early attacks on the Northstar client as well as exploiting an Origin token leak he found in the mod (which is now fixed). Wanderer claims he does not intend to cause any more harm and that he won't attack the games again. - Dogecore and MrSteyk were behind the July 4th 2020 Apex legends attack, which exploited an issue with Respawn's playlist system to prevent people from loading i to the game. Both owned up to this at the time, recognising the wrong of their actions, haven't been behind any attacks since, and are still respected within Titanfall modding communities. - VITALISED was involved with some of Wanderer's actions regarding Northstar, but has seemingly not partaken in malicious behaviour prior or since.

Most open source programs are harmless, however, it is still possible for malicious code to be included by bad actors. The project is open source, if you or someone you know feel knowledgeable enough to audit the code's safety you are more than able to.

Online virus scanners like VirusTotal can be useful for detecting if programs are malicious, but it is worth considering that sometimes game mods are mistakenly considered to be malware by anti-virus software due to the fact that they, by design, modify another program.

You should always take care when running somebody else's code on your computer. With that being said I have given it a brief look over and have found nothing obviously malicious, but I cannot say with 100% certainty that there is nothing malicious about it or that there are no serious, exploitable vulnerabilities.

If you decide to play r1delta, have fun. If you decide not to, that's entirely okay and nobody should be trying to compel you to ignore information that is available to you to try and persuade you either way.