The popularity of key phrases may decrease, and in this case, previously added phrases get the status «Low search volume». Here Google writes about this in more detail — https://support.google.com/google-ads/answer/2616014.

In order not to be distracted by their maintenance, you need to delete them. I wrote a script that does this automatically:

function main() {
    var key_ids_arr = [];
    // Select keywords with the status "Low search volume", active, in active groups
    // in active campaigns, with 0 impressions in the last 30 days
    var search = 'SELECT ad_group_criterion.criterion_id, ad_group.id ' +
        'FROM keyword_view ' +
        'WHERE ad_group_criterion.system_serving_status = "RARELY_SERVED" ' +
        'AND ad_group_criterion.approval_status = "APPROVED" ' +
        'AND ad_group_criterion.status = "ENABLED" ' +
        'AND ad_group.status = "ENABLED" ' +
        'AND campaign.status = "ENABLED" ' +
        'AND metrics.impressions = 0 ' +
        'AND segments.date ' +
        'DURING LAST_30_DAYS LIMIT 50000';
    var report = AdsApp.search(search, {
        apiVersion: 'v8'
    });
    while (report.hasNext()) {
        var row = report.next();
        var key_id = row.adGroupCriterion.criterionId,
            ad_group_id = row.adGroup.id;
        // collecting their IDs
        key_ids_arr.push([
            ad_group_id,
            key_id
        ]);
    }
    if (key_ids_arr.length > +0) {
        var keywordsIterator = AdsApp.keywords()
            .withIds(key_ids_arr)
            .get();
        while (keywordsIterator.hasNext()) {
            var keyword = keywordsIterator.next();
            // delete
            keyword.remove();
        }
    }
}