When it was allowed, was the practice of Satī a.k.a sahagamana meaning 'going with' or sahamaraṇa ('dying with') limited to one particular varṇa, the kṣatriya?
Or was it applicable to brāhmaṇa and other women as well?
P.S. I couldn't find the answer under the more general 'What is the truth behind the practice of Sati? How is it related to Hinduism?' so asking it as a separate question.
It was/is applicable to women of all Varnas except pregnant women and women who have young children.
Practice of Sati was always voluntary in past, although the practice changed a lot due to many historical reasons. Women who escape sati practice by fainting need to observe Prajapatya vow.
Chapter 4, Preta Khanda of Garuda Purana mentions it in detail (translated by MN Dutt)
योषित्पतिव्रता या स्याद्भर्तारं यानुगच्छति ८९
प्रयोग पूर्वं भर्तारं नमस्कृत्यारुहेच्चितिम्
चितिभ्रष्टा तु या मोहात्सा प्राजापत्यमाचरेत् ९०89(b)-90.A woman who has been chaste and faithful to her husband should mount on the pyre after bowing to her (deceased) husband before the funeral rites start. One who gets away from the pyre due to fainting should observe the vow named präjäpatya.
तिस्रः कोट्योर्धकोटी य यानि लोमानि मानुषे
तावत्कालं वसेत्स्वर्गे भर्तारं यानुगच्छति ९१91.One who ascends the pyre and follows up her husband stays in heaven for a period equal to the number of hair on the body, three and a half crore.
व्यालग्राही यथा व्यालं बिलादुद्धरते बलात्
तद्वदुद्धृत्य सा नारी तेनैव सह मोदते ९२92.Just as the snake-charmer takes out the snake from the hole so also she takes out her husband from hell and enjoys with him in paradise.
तत्र सा भर्तृपरमा स्तूयमानाप्सरोगणैः
क्रीडते पतिना सार्धं यावदिन्द्रा श्चतुर्दश ९३93.She who ascends the pyre goes to heaven. She is praised by the celestial nymphs and enjoys with her husband so long as the fourteen Indras rule in heaven successively.
ब्रह्मघ्नो वा कृघ्नो वा मित्त्रिघ्नो वा भवेत्पतिः
पुनात्यविधवा नारी तमादाय मृता तु या ९४94.Even if the man has killed a brähmana or a friend or any other person of noble conduct he is purified of sins by his wife who ascends his pyre.
मृते भर्तरि या नारी समारोहेद्धुताशनम्
सारन्धतीसमाचारा स्वर्गलोके महीयते ९५95.A woman who enters fire after the death of her husband prospers in the heaven like Arundhati.
यावच्चाग्नौ मृते पत्यौ स्त्री नात्मानं प्रदाहयेत्
तावन्न मुच्यते सा हि स्त्रीशरीरात्कथञ्चन ९६96.Until and unless the woman burns herself after her husband's death she is never released from the bond of her sex.
मातृकं पैतृकं चैव यत्र चैव प्रदीयते
कुलत्रयं पुनात्येषा भर्तारं यानुगच्छति ९७97.A woman who follows her husband purifie the three families on her mother's side, the three families on her father's side and the three families on her husband's side.
आर्तार्ते मुदिते हृष्टा प्रोषिते मलिना कृशा
मृते म्रियेत या पत्यौ सा स्त्री ज्ञेया पतिव्रता ९८पृथक् चितां समारुह्य न प्रिया गन्तुमर्हति
क्षत्त्रियाद्याः सवर्णाश्च आरोहेयुरपीह ताः ९९चाण्डालीमवधिं कृत्वा ब्राह्मणीतः समो विधिः
अगर्भिणीनां सर्वासामबालताक्मेनामपि १००98-100. That woman is chaste who is sad when her husband is sad, who is glad when her husband is glad and who pines when he is out of station and dies when he is dead. Common rites are enjoined for all women right from brähmani to Cändäli except for those who are pregnant or who have young children.
As shown in this answer the argument that "Sati does not apply to a Brahmin's widow", is one of the objections posed by the objectors and which the commentator Mitaksra refutes as follows:
Objection:
An objection:—The rule of Sati does not apply to Brahmana widows.—But there are texts (says an objector) which prohibit (Anugamana) (satism) for a Brahmani woman, such as :—" There is no anugamana or self-immolation for Brahmani woman ; for this is the command of Brahma. But among the other castes this anugamana is said to be the highest austerity. (Their duty is) to do their husband's good, while he is living ; and to commit suicide when he is dead. But that woman of the Brahmana caste, who follows her dead husband (by anugamana) does not lead either herself or her husband to heaven because of the sin of her suicide.
Mitaksara's reply:
Reply,—To this we reply that these and several other texts, relate to the ascending of a separate funeral pile, because of this special Smriti : —;" A Brahmana woman cannot follow her husband by ascending a separate funeral pile." From this it follows, that the women of the Ksatriyas and the rest are allowed to ascend a separate funeral pile.
