I believe that according to the historical record, most cases have been forced, for some reason or another: if not outright coerced, then pressured. Is this correct?

If so, then the line between a sovereign who has abdicated the throne or been deposed is a fine one -- especially if (as just hinted above) it's not always clear as to whether the abdication itself was voluntary or coerced.

It's only in modern history where one has witnessed sovereigns who are aging, or who feel they have served their countries long enough, voluntarily stepping down in favor of their heirs -- believing that the said countries would be better served by younger people. And it seems that many of these monarchies draw upon foreign examples for inspiration.

There simply is NO way NOT to see the Grand Duchess Charlotte of Luxembourg acting under the influence of her fellow Benelux colleague, Queen Wilhelmina of the Netherlands, when in the 1960's she stepped aside in favor of her son, who thus (like Queen Juliana of the Netherlands) was able to gain the throne early.

Otherwise, Jean would have had to wait until the age of 64 (nearly a senior citizen), before finally becoming the reigning grand duke -- just as his Dutch counterpart would have had to wait until becoming a middle-aged woman of 53, before ascending the throne.

As such, Wilhelmina clearly set the precedent for this practice -- that is, in MODERN royal history (not just in Europe but also in Japan). However, she herself had drawn upon a much earlier example: that of Holy Roman Emperor Charles V.

Have there been other examples in the more distant past of abdications which were (entirely) voluntary -- where the sovereigns simply wanted to retire?

This clearly does not characterize the earlier Benelux abdications in history (King Willem I of the Netherlands in 1840, Grand Duchess Marie Adelaide of Luxembourg in 1919, and King Leopold III of the Belgians in 1950).