I'm going to pull apart your question a little bit, since you've given no follow-up text.

First, it's really necessary to provide some context on the passage you mention. When we situate Dt 20 against Ancient Near East warfare practices it is not so unusual. Both Egypt, for example under Thutmose III (1490-36), and the Hittites provide examples of 'scorched earth' policies, enslaving whole populations, and razing the city and its flora.

Within the Deuteronomy passage you cite, it's noteworthy (a) that there is first an offer of peace, (b) that male citizens were for the great part combatants and so military targets, (c) that absent the entire male population the only option for women and children is absorption into another people group, (d) there are further laws relating to proper treatment for woman taken as wives during war (21:10-14), (e) the final instruction regarding trees is a prohibition of scorched earth policy which is remarkable in its constraint.

Secondly, you ask when a 'Judeo-Christian' army last implemented this. 'Judeo-Christian' is a sometimes useful term when pointing out commonalities of belief between Judaism and Christianity, but it doesn't really make any sense in this context, there are no Judeo-Christian armies. I suppose what you mean is "Jewish or Christian" armies. Even this is problematic, since, apart from the Crusades, Christians have generally not fielded armies based on religious affiliation. I would contest that the overlap in national and religious identity in pre-Christian Judaism means that trying to identify an army as 'Jewish' on religious grounds is also problematic.

Thirdly, the question rests on a general misunderstanding about how Christians have generally read and applied the Old Testament Law. Across the three major traditions (Catholic, Protestant, Orthodox) it is fair to say that while individual aspects may vary, the OT Law is recognised as a temporal and provisional law given to the nation of Israel, and is not binding as a legal code for Christians. Almost no Christian group ever (almost, there are some branches of American Theonomy that I exempt) has sought to live in accordance with the OT Law code, so there is no real reason to expect any 'Christian' army to implement these stipulations.

Rather, what you see in the development of Christian theology is the emergence of a doctrine of Just War that is partly born out of a philosophy of Natural Law alongside a reading of principles from the Bible.

That leaves your question open - to what extent did ancient Israelites conform their war-practices to the Deuteronomic code? I would prefer to leave that part of the question to someone more specialised in that particular field.