pnpm doesn't execute lifecycle scripts by default, so it avoids the particular attack vector of "simply downloading and installing an NPM package allows it to execute malicious code."
As phiresky points out, you're still "download[ing] arbitrary code you are going to execute immediately afterwards" (in many/most cases), so it's far from foolproof, but it's sufficient to stop many of the attacks seen in the wild. For example, it's my understanding that last month's Shai-Hulud worm depended on postinstall scripts, so pnpm's restriction of postinstall scripts would have stopped it (unless you whitelist the scripts). But last month's attack on chalk, debug, et al. only involved runtime code, so measures like pnpm's would not have helped.
As phiresky points out, you're still "download[ing] arbitrary code you are going to execute immediately afterwards" (in many/most cases), so it's far from foolproof, but it's sufficient to stop many of the attacks seen in the wild. For example, it's my understanding that last month's Shai-Hulud worm depended on postinstall scripts, so pnpm's restriction of postinstall scripts would have stopped it (unless you whitelist the scripts). But last month's attack on chalk, debug, et al. only involved runtime code, so measures like pnpm's would not have helped.