Is it permissible for a woman to conceive a child with a man she fears might leave her, and what is the ruling of Islamic law on that?

It is not permissible to have children with a foreign man, as this involves major sins due to fornication. Relationships between a man and a foreign woman are forbidden by Islamic law and must be repented from, and God's commands must be obeyed.

Whoever has fallen into this sin must repent to God and increase good deeds, such as prayer, fasting, and charity.

It is advisable to marry this person to legitimize the relationship; otherwise, the relationship must be severed immediately.

However, if the question is about having children with one's husband to avoid divorce, there is no harm in that, as it is a shared right between the spouses.