Where should a question on Logic be posted? How does a querier decide?
Or should there be a separate SE for Logic?
The questions quoted here appear admissible and suitable also for Philosophy SE?
Questions on logic are welcome here to the extent that they are "mathematical" questions. There is a tag logic which has about 8,500 questions at the moment.
Questions that are almost entirely mathematical, such as technical questions on set theory, proof theory, computability theory, model theory, etc. that would be resolved with mathematical reasoning, are very much on topic here. Of course, these questions may relate to philosophical topics, and the best answers may include these relationships in addition to the mathematics. But the main purpose of a question on this site should be mathematics, rather than philosophy.
Questions that are not amenable to mathematical techniques are not on topic here. These might include questions on pure philosophy or questions that are purely historical.
For questions that are on-topic both here and on Philosophy.SE, you will likely get very different answers on the two sites, in much the same way that logic textbooks for philosophers are different from logic textbooks for mathematicians. This does not mean, however, that the same question should be posted to both sites - that kind of "cross posting" is discouraged.