This is one of the mitigations for speculative execution CPU bugs. It was done as part of qubes-secpack/qsb-043-2018.txt at master · QubesOS/qubes-secpack · GitHub, but applies to several other later bugs too. Generally, it is not safe to re-enable SMT unless you really understand the consequences and apply additional measures relevant to your use case.
In any case, if you really know what you are doing, you can switch it on by keeping the existing line unchanged, but adding another similar one with smt=on. But note, such configuration is not security supported - there may be cases where some issue (current or future) is mitigated by smt=off on Qubes, but if you have it changed, you’ll be vulnerable. If you do this, you need to keep track of such issues yourself, according to your threat model. That said, it may be perfectly fine in some cases, where isolation between VMs isn’t really necessary for given use case (like - purely testing / demo system).