In this answer it states that Brahmins should not perform vedic rituals for Shudras. Question is: does this mean most of the brahmins who are in temple or in priest role are breaking the rules by performing puja/havan for Shudras? These days there are very few Kshatriya/Vaishyas; so it is generally Brahmins or Shudras. So, how this works? Any views?
The answer you link to quotes references from dharmaśāstra and dharmasūtra literature. These are smṛti and not śruti, hence they are not authoritative for all times and places. This is why different dharmaśāstras and dharmasūtras were written in different epochs and different places, and in fact new ones can and should yet be written.
As I understand, Hinduism is not in any case a religion of absolute commandments. Rather, the right thing to do in any situation according to dharma should be determined by careful thinking (samīkṣā) and group discussion. This is the message of Mahābhārata and a lot of Indian literature.
Hence even in dharmaśāstra, there is variability on this question - I am quoting Kane History of Dharmaśāstra 2.1 p.156-7 as follows - "at least one ancient teacher was found (Bādari) was found who advocated that even śūdras could perform the Vedic sacrifices The Bhāradvāja Śrauta sūtra (V.2.8) states the opinion of some that the śūdra can consecrate the three sacred Vedic fires."