Does Deuteronomy 22:25-29 imply that the rape of an unbetrothed girl is not a death penalty offense?
Quick answer: no. Rape is always a mandatory death penalty offense. Scripture likens this crime to murder, which also a mandatory death penalty offense.
There are actually a couple of issues implicit in this question. The first issue is that certain Bible translations mistranslate a Hebrew word in Deut. 22:28 as "rape." This problem is discussed in the Question/Answer Does Biblical law require a girl to marry her rapist?.
Apart from mistranslations, the reason this question might come up is that scripture states that there is a death penalty for raping a "betrothed woman", but it does not explicitly specify a penalty for raping an unbetrothed woman. Here is the relevant scripture passage:
25 But if the man finds the lady who is pledged to be married in the field, and the man forces her and lies with her, then only the man who lay with her shall die; 26 but to the lady you shall do nothing. There is in the lady no sin worthy of death; for as when a man rises against his neighbor and kills him, even so is this matter; 27 for he found her in the field, the pledged to be married lady cried, and there was no one to save her. Deuteronomy 22:25-27WEB
Biblical case laws are often written with the intent of distinguishing legally-relevant differences between cases. Sometimes, though, the cases are "entangled" in a way that defies a superficial reading. For example, if you do not understand that Deut. 22:13-21 is dealing with two separate legal cases (slander and lying about one's state of virginity), then you might (mistakenly) conclude that the girl involved is considered guilty until she proves herself innocent (See Is the premarital unchastity case of Deut. 22:13 an example of the justice system assuming guilt until a defendant proves her innocence?)
In this section of the law, the cases are intended to establish that a woman's claim that she "cried out" in a field or other unpopulated area is sufficient to protect her from accusations of adultery (which is the only reason why the word "betrothed" is there in the case). The rapist must be put to death, but the girl has done "nothing worthy of death" (v. 26).
Rape of an unbetrothed girl is, a fortiori, a mandatory death penalty offense, based upon the general equity of Deut. 22:25-26. In v. 26, rape is likened to murder, an act in which the primary (human) victim is the person who is killed. It is reasonable to believe that the primary victim in an act of rape is the woman (not a betrothed husband). The woman's state of betrothal is irrelevant to this comparison. Therefore, when scripture equates rape to murder in v. 26 (which is also a mandatory death penalty offense), the relevant similarity between the crimes shows that (a fortiori) the rape of an unbetrothed girl also mandates the death penalty (as long as there are two or more witnesses: Deut. 19:15).